The present invention relates generally to a television receiver, and in particular to a mobile television receiver.
Unlike stationary television receivers, mobile television receivers encounter constantly changing reception conditions. For example, when a mobile television receiver is installed in a vehicle, the position of the antenna changes with the position of the vehicle. In contrast to an antenna of a stationary television receiver which can be optimally aligned and fixed once, a mobile television receiver and its antenna will be located in many positions relative to the transmitter, resulting in a continually changing reception. For this reason, during the development of mobile television receivers, special attention has been paid to the response of the television receiver given low levels of reception.
A low level of reception indicates a high noise component in the useful signal. The consequences of a small signal/noise ratio manifest themselves in a subjectively barely discernable picture, loss of horizontal or vertical synchronization, onset of color suppression, or even complete loss of picture. This response is essentially caused by the unlocking of the phase-locked loops (PLL) for video signal processing, by unsteady synchronous pulse separation, by disturbed automatic gain controls (AGC), and defective color subcarrier recovery.
For example, should the synchronous demodulator of the frequency phase-locked loop (FPLL) unlock while the intermediate frequency is being converted to baseband, a complete loss of picture will occur. In addition, a loss of lock causes an extended dead period for the entire system, since each phase-locked loop requires a certain period of time to return to a new lock-in state.
In those applications in which the received video signal is utilized for subsequent transmission in a multimedia system, a compression method (e.g., MPEG) is generally employed to reduce the transmission bandwidth. The algorithms used in such compression methods reduce the transmitted information, depending for example on the video information and its modifications. Video content with a uniform form thus requires a lower bit rate for transmission, while a more detail-rich video content requires a higher one. However, the noise components in the video signal are interpreted by the compression algorithm as detail-rich image modifications.
Therefore, there is a need for a television receiver having improved performance during mobile use.